Flexible externally toothed gears, which are component parts of strain wave gearings typically used as reducers, are known to have flattened-cylinder, cup-shaped, or top-hat-shaped configurations. JP-A 2006-57684 discloses a flat strain wave gearing, JP-A 2005-69402 discloses a cup-shaped strain wave gearing, and JP-A 2009-156462 discloses a top-hat-shaped strain wave gearing.
The flexible externally toothed gears for these strain wave gearings are manufactured using specialty thermal-refined steel having exceptional mechanical strength. The surface hardness of thin-walled flexible externally toothed gears needing highly precise machining is HRC30-HRC50.
WO 2011/122315 proposes a method for manufacturing a wave gear base material used in the manufacture of a flexible externally toothed gear. In the manufacturing method described in this document, it is disclosed that a flexible externally toothed gear is manufactured through steps for hot-forging, rotary cutting, gear cutting, and shot-peening. It is also disclosed that steel having a carbon content of 0.5% or less is used as the metal material. It is furthermore disclosed that a primary molded article formed from the steel is heated before being rapidly cooled to a temperature higher than the martensite start temperature (Ms temperature) so that the main phase of the metal structure will be bentonite, or that the primary molded article is heated before being rapidly cooled to the martensite region and then tempered so that the main phase of the metal structure will be sorbite. It is also disclosed that it is possible to stably obtain, as a wave gear base material, a primary molded article having excellent properties in terms of both the necessary spring characteristics and ductility and the cutting and machining properties preferred for gear cutting and machining.
Patent Document 1: JP-A 2006-57684
Patent Document 2: JP-A 2005-69402
Patent Document 3: JP-A-2009-156462
Patent Document 4: WO 2011/122315